It's a good thing that he wants to see his mom. It means his mom is a good mother.
When a child says that to me, I say something like, "Yes. She's a really nice lady. What would you do if you were with her?" The child then thinks of the things they would do and talks about it and that usually calms them down. If not, I try to gently talk them down somehow.
I don't know if this was you that asked in another forum, but I replied to a similar question on a Montessori board today. I'll say the same thing here. Basically, one of the best things the school can do is set up a ritual for the child. I had one student a few years ago who cried every morning. I finally watered plants in front of him and asked if he wanted to do it. He said no for a while, then finally decided to say yes and took to doing it. He cried a little bit while he was doing it at first, then came in and was looking forward to watering the plants. Many children do not know what to expect when they come to school every morning and many children think it might be different every time. Providing a ritual for the child helps him or her know, "Every morning, when I go in, I'm going to (straighten the shelves, set up the snack, water the plants, tear off the calendar page...etc.)"
I assume you think this is a case of your son simply not being ready or something similar. I mean I hope you evaluated that this is not a situation that is dangerous for him.